Method to remove unwanted, unexposed, positive-working, radiation-sensitive layer

Abstract

The presence of unwanted ink-receptive sections on developed positive-working lithographic plates due to shading by plate-holding clamps during imaging, is avoided by the use of clamps that are substantially transparent to the imaging radiation. In the case where the plate precursors can be imaged by heat, the presence of unwanted unexposed sections can also be avoided by selectively heating those sections shaded by the clamps, while avoiding heating the image section of the plate.

What is claimed is:  
     
       1. A method for eliminating an unwanted ink-receptive section of a printing plate precursor present after treatment with a developer, said plate precursor comprising a heat-sensitive layer capable, upon treatment with a developer following exposure to imaging radiation, of generating ink receptivity in a developed plate prepared from said precursor, said unwanted ink-receptive section being a shaded section of said heat sensitive layer shaded by a plate-holding element from exposing radiation during an imagewise exposure of said plate precursor and therefore remaining unexposed to said exposing radiation following said imagewise exposure, the method comprising heating said shaded section of said plate prior to developing said plate precursor to a temperature such that following development said shaded section is hydrophilic wherein said heating occurs during said imagewise exposure. 
     
     
       2. The method according to  claim 1  wherein said plate precursor is a positive working plate precursor and wherein said heating of said shaded section renders said shaded section hydrophilic by rendering said heat-sensitive layer of said shaded section soluble or dispersible in said developer. 
     
     
       3. The method of  claim 1  wherein said unwanted shaded section comprises a photothermal conversion material. 
     
     
       4. A method for eliminating an unwanted ink-receptive section of a printing plate precursor present after treatment with a developer, said plate precursor comprising a heat-sensitive layer capable, upon treatment with a developer following exposure to imaging radiation, of generating ink receptivity in a developed plate prepared from said precursor, said unwanted ink-receptive section being a shaded section of said heat sensitive layer shaded by a plate-holding element from exposing radiation during an imagewise exposure of said plate precursor and therefore remaining unexposed to said exposing radiation following said imagewise exposure, the method comprising heating said shaded section of said plate prior to developing said plate precursor to a temperature such that following development said shaded section is hydrophilic, wherein said plate-holding element is also a heating element and the step of heating said section shaded by said plate-holding element is performed with said heating element. 
     
     
       5. A method for preparing a printing plate comprising the steps of: 
       (a) providing a positive-working printing plate precursor comprising a radiation-sensitive region on a front surface of said precursor, said radiation-sensitive region comprising a heat-sensitive layer capable, upon treatment with a developer, of generating ink receptivity in a developed plate prepared from said precursor;  
       (b) mounting said precursor on an exposure device with a plate-holding element wherein said element overlaps a section of said radiation-sensitive region of said precursor and wherein said element is also a heating element;  
       (c) imagewise exposing said radiation-sensitive region of said precursor to exposing radiation, said radiation selected to render said heat-sensitive layer exposed to said radiation incapable of generating ink receptivity in said developed plate upon treatment of said precursor with said developer;  
       (d) using said heating element to heat said section of said radiation-sensitive region overlapped by said element, said heating selected to render said heat-sensitive layer of said section incapable of generating ink receptivity in said developed plate upon treatment of said precursor with said developer;  
       (e) removing said precursor from said exposure device; and  
       (f) developing said precursor in said developer to form said plate.  
     
     
       6. The method of  claim 5  wherein step c) is performed simultaneously with step (d). 
     
     
       7. The method of  claim 5  wherein step (c) is performed after step (d). 
     
     
       8. A method for preparing a printing plate comprising the steps of: 
       (a) providing a positive-working printing plate precursor comprising a thermally imageable region on a front surface of said precursor, said thermally imageable region comprising a heat-sensitive layer;  
       (b) determining an area on said thermally imageable region that will be undesirably shaded during imagewise exposure;  
       (c) heating said undesirably shaded area of said thermally imageable region, said heating selected to render said undesirably shaded area soluble or dispersible in said developer;  
       (d) subsequently mounting said precursor on an exposure device with a plate-holding element wherein said element overlaps said heated area of said thermally imageable region of said precursor;  
       (e) imagewise exposing said thermally imageable region of said precursor to imaging radiation, said radiation selected to render said heat-sensitive layer exposed to said radiation soluble or dispersible in a developer;  
       (f) removing said precursor from said exposure device; and  
       (g) developing said precursor in said developer to form said plate.  
     
     
       9. A method for preparing a printing plate comprising the steps of: 
       (a) providing a positive-working printing plate precursor comprising a thermally imageable region on a front surface of said precursor, said thermally imageable region comprising a heat-sensitive layer;  
       (b) mounting said precursor on an exposure device with a plate-holding element wherein said element overlaps said heated area of said thermally imageable region of said precursor;  
       (c) imagewise exposing said thermally imageable region of said precursor to imaging radiation, said radiation selected to render said heat-sensitive layer exposed to said radiation soluble or dispersible in a developer;  
       (d) removing said precursor from said exposure device;  
       (e) determining an area on said thermally imageable region that was undesirably shaded during the imagewise exposure;  
       (f) heating said undesirably shaded area of said thermally imageable region, said heating selected to render said undesirably shaded area soluble or dispersible in said developer; and  
       (g) developing said precursor in said developer to form said plate.